U.S. Attorney General Jeff Sessions was fired on Wednesday after receiving unrelenting criticism from President Donald Trump for recusing himself from an investigation into Russia's role in the 2016 presidential race.

In a step that could have implications for the investigation led by Special Counsel Robert Mueller, Trump replaced Sessions with Matthew Whitaker, who will be acting attorney general. He had been Sessions' chief of staff.

The top Democrat in the U.S. Senate immediately called on Whitaker to recuse himself from the Mueller probe.

We are pleased to announce that Matthew G. Whitaker, Chief of Staff to Attorney General Jeff Sessions at the Department of Justice, will become our new Acting Attorney General of the United States. He will serve our Country well....

“Given his previous comments advocating defunding and imposing limitations on the Mueller investigation, Mr. Whitaker should recuse himself from its oversight for the duration of his time as acting attorney general,” Senate Democratic leader Chuck Schumer said in a statement.

In an opinion piece for CNN that appeared on Aug. 6, 2017, while he was a commentator for the network, Whitaker, a former U.S. attorney, said Mueller would be crossing a line if he investigated the Trump family's finances. The piece was titled: “Mueller’s investigation of Trump is going too far.”

A spokesman for the special counsel’s office declined to comment on Sessions’ resignation and what it means for Mueller’s probe.

Trump announced Sessions' departure on Twitter and thanked him for his service. Sessions said in a letter to Trump that he had resigned at the president's request.

Sessions' exit had been widely expected to come soon after Tuesday's congressional elections, in which Republicans retained their majority in the Senate but lost control of the House of Representatives.

Never in modern history has a president attacked a Cabinet member as frequently and harshly in public as Trump did Sessions, 71, who had been one of the first members of Congress to back his presidential campaign in 2015.

Democratic Representative Jerrold Nadler, expected to chair the House Judiciary Committee starting in January, demanded answers in a tweet about Trump's reasons for firing Sessions.

"Why is the President making this change and who has authority over Special Counsel Mueller’s investigation? We will be holding people accountable," Nadler asked on Twitter.

Mueller's probe, operating under the auspices of the Justice Department, already has yielded criminal charges against several Trump associates and has clouded his presidency for many months.

Republicans had repeatedly urged Trump not to oust Sessions, a former conservative Republican senator from Alabama, before the elections lest it create political fallout.

They had also argued that Sessions should be allowed a graceful exit after he doggedly carried out Trump's agenda on illegal immigration and other administration priorities.

RECUSAL OVER RUSSIA

Trump was only a few weeks into his presidency in March 2017 when Sessions upset him. Rejecting White House entreaties not to do so, Sessionsstepped aside from overseeing the FBI's probe of potential collusion between the Trump presidential campaign and Moscow. Sessions cited news reports of previously undisclosed meetings he had with Russia's ambassador to Washington as his reason for recusal.

Rosenstein took over supervision of the Russia investigation and appointed Mueller in May 2017 as the Justice Department's special counsel to take over the FBI's Russia probe after Trump fired FBI Director James Comey.

A permanent replacement for Sessions must be confirmed by the U.S. Senate, which Trump's Republicans will continue to control as a result of Tuesday's midterm elections.

Mueller is pursuing an investigation into whether Trump's campaign colluded with Russia, whether Trump unlawfully tried to obstruct the probe, and possible financial misconduct by Trump's family and associates. Mueller has brought charges against Trump's former campaign chairman and other campaign figures, as well as against 25 Russians and three firms accused of meddling in the campaign to help Trump win.

Trump has denied his campaign colluded with Russia.

Trump publicly seethed over Sessions' recusal and said he regretted appointing him. On Twitter, he blasted Sessions as "VERY weak" and urged him to stop the Russia investigation. In July 2017, he told the New York Times that if he had known Sessions would recuse himself, he never would have appointed him attorney general.

Washington Post journalist Bob Woodward wrote in his book "Fear" that Trump, talking to a White House secretary, disparaged Sessions as "mentally retarded" and a "dumb Southerner" while mocking his accent.

There were news reports in the weeks after Mueller's appointment that Sessions had offered to resign. Sessions usually remained quiet on Trump's criticism, but defended himself in February 2018 after a Trump tweet criticizing his job performance by saying he would perform his duties "with integrity and honor."

RESPONDING TO TRUMP

In August, Sessions punched back harder after Trump said in a Fox News interview that Sessions "never took control of the Justice Department."Sessions issued a statement saying he "took control of the Department of Justice the day I was sworn in" and vowed not to allow it to be "improperly influenced by political considerations."

As for his own involvement with Russia, Sessions was questioned in January by Mueller's team and has offered shifting public accounts. He has said nothing improper transpired in his meetings during the campaign with Ambassador Sergei Kislyak. In congressional testimony in November, he said he now recalled a meeting during the 2016 campaign in which a campaign adviser, with Trump present, offered to use connections with Moscow to arrange a meeting between Trump and Russian President Vladimir Putin.